More particularly, the invention relates to a method of measuring the color distribution of the surface of a spherical vegetable or fruit, such as an apple, a pear, a tomato, a paprika, or an eggplant, so as to enable assessment of the of its color distribution.
Still more particularly, the invention relates to a method of automatically sorting vegetables or fruits on the basis of the color distribution of those vegetables or fruits.
In the art, many methods are known for automatically sorting vegetables or fruits on the basis of color. U.S. Pat. No. 4,106,628, for instance, discloses an apparatus in which each item passes two optical sensors arranged on opposite sides of a conveyor, each detector issues a signal which is representative of the color detected and the two detected color signals are averaged. A disadvantage of this apparatus is that the items must be transported in separate rows, which requires a system of two detectors for each separate row. A further disadvantage of this known apparatus is that it only provides an indication on the color of two opposite surface portions of the item, which colors, moreover, are averaged, while the further surface of the item may deviate strongly from the measured surface portions.
Accordingly, it is an object of the invention to provide a method of the above-mentioned type, in which the entire surface of the item to be measured is detected.
To that end, in a method according to the invention, use is made of a camera and the items are subjected to a rotation of at least 360.degree. within the field of view of the camera.
Also known in the art are methods for evaluating and automatically sorting items using a camera, in which the items are subjected to a rotation of at least 360.degree. within the field of view of the camera. U.S. Pat. No. 5,030,001, for instance, discloses a method for evaluating eggs, in which a gray value is determined for each surface element of an egg, the number of surface elements that have a given gray value are added, and the magnitude of any surface defect is determined on the basis of that addition. This method, however, is not suited for determining the color distribution of a fruit or a vegetable.
It is known in practice that the degree of ripeness of a fruit or a vegetable can be derived from the color of that fruit or vegetable. For instance, anyone will recognize a green tomato as being unripe and a red one as ripe. Between the stages from unripe to ripe, however, a fruit or vegetable will go through different stages of ripeness, in which the tomato of the cited example, for instance, is partly red and partly green. Especially in the vegetable and fruit trade, for instance in evaluating a purchase or in evaluating whether a certain shipment will "survive" transportation abroad, it is desirable that a fairly accurate estimate can be made of the number of days that a product will keep or remain marketable, i.e., it is desirable that its ripeness can be assessed.
At present, in practice, the degree of ripeness is evaluated by so-called inspectors, who assess the ratio of the colors of a product by sight and on that basis judge its ripeness. A disadvantage of such a method is that it is very labour-intensive if it is desired that each product be assessed individually. It will further be clear that such a method of assessment incorporates a substantial element of subjectiveness.
There is accordingly a need for a method and an apparatus for automatically and objectively assessing the degree of ripeness of a fruit or vegetable.
European patent specification 0.105.452 discloses a method for sorting fruit, which also gives an indication of the degree of ripeness of some surface portions of a fruit to be examined. The items to be examined are conveyed in rows past a detection unit and the item to be examined is scanned to form a picture of a linear surface portion of the item, which picture consists of a predetermined number of picture segments. The detection unit comprises a separate detector for each picture segment, while the item to be examined is rotated in the field of view of the detection unit about an axis parallel to the linear surface portion so as to enable the entire surface of the item to be scanned. The measured data of the entire surface of that item are stored in a memory of a computer to be processed further.
A disadvantage of this apparatus is that the items must be transported in separate rows, requiring a detection unit for each row. Further, the detection unit can scan only one item at a time, which has a limiting influence on the processing capacity of the detection unit and hence of the transport and sorting system in which such a detection unit will be utilized.
Further, each detector provides only one analog value for each picture segment, namely, a gray value (number).
A further disadvantage of this known method is that a relatively large memory is required for collecting the measured data. If the number of picture segments in the linear surface portion is represented by D and the number of line scan cycle during a complete rotation of the item to be examined is represented by N, the known method requires a memory of 2.times.N.times.D locations to derive from the measured data an indication on the occurrence of blemishes on the surface of the item to be examined. For providing information on the ripeness of a fruit or vegetable to be examined, the known method requires an additional four memory locations for each picture segment, as well as two color filters and two detectors.
A further problem associated with this known method is that during measurement the light rays coming from the item strike the detectors at a varying angle.